The method which is the subject of the present invention is in the field of methods for the production of drinking water for communities. It relates more particularly to the field of membrane filtering methods of the ultrafiltration type.
Conventionally, there are known water purification devices, based on a sand bed upon which the raw water is added. The water passes through the sand bed and is retrieved under the sand freed from a great number of particles, pollutants, etc. These devices have been known for very many years and are particularly widespread in certain countries for the cleansing of waste town water. The United States and Holland can be mentioned in particular.
Most of these devices, dating from tens of years ago or even from more than a century ago, must now be replaced by systems with better performance. In fact they do not all make it possible to achieve water complying with present quality standards at the purifier outlet.
A conventionally offered solution consists in destroying the existing installations and replacing them completely with a new construction adapted to a water purification method of different type, whether by ultrafiltration or another method. It is clear that this replacement is very expensive for communities, both with regard to the new unit construction costs and to the cost of destruction of the previously existing units.
In the prior art there are also known devices using the height of water above membranes to create a pressure difference sufficient for the filtration. Among these documents, a KURITA device described in the Japanese patent abstract Vol. 012 No 155, May 12, 1988, effectively uses the height of water as a pressure generator but in a very complex device, partially mobile in rotation, with superimpositions of long fibers disposed horizontally and potted at their two ends.
Another document (KUBOTA, Japanese patent abstract Vol. 96 No 008, Aug. 30, 1996) relates to a device for treating waste water by membranes placed vertically, the potting being formed by the water, the filtered water being retrieved from the top, with the need for a great height of water in the tank.
This device necessitates an injection of air between the membranes in order to allow correct operation by agitating the waste water around the membranes.
In yet another document (PREUSSAG, patent application DE 296 20 426 U), the fibers are disposed horizontally and potted at their two ends, a vacuum pump being used to create a pressure difference with the waste water tank. A flow of air is necessary in order to agitate the effluents. The retrieval of the filtered water is carried out at the top of the group of membranes, which increases the height of water necessary for the treatment.
These various devices are complex and cannot be installed easily in existing sand bed water treatment installations.
The present invention therefore proposes overcoming these disadvantages by proposing a new method of water purification by filtration.
The purpose of the invention is also to propose a sand bed purifier rehabilitation method which is economical and fast to implement.
According to another objective of the invention, the rehabilitated unit makes the greatest possible use of a large number of elements from pre-existing units, and in particular of the pipes and devices for separating sludges and retrieved clean water.
According to yet another purpose of the invention, this method takes advantage of certain specific arrangements of sand silo purifiers in order to simplify the regulation of the new purifier, thus producing an operational simplification and therefore a saving of labor, regulation equipment and maintenance.
The method according to the invention is therefore a method of water filtration using immersed membranes, of the ultrafiltration membrane type, the filtration through the membranes being carried out using, as a source of differential pressure, the height of water present in the basin in which the membranes are immersed and are of the fibre type with an outer skin, potted at the low point of the said membranes, characterized in that the membranes are disposed in cylindrical containers.
It is understood that this arrangement, of the “filtering floor” type, allows a simple and effective regulation of the filtration pressure, which devices based on pumps do not allow. The situation here is one of operation at constant pressure and with variable flow rate, unlike the existing systems which operate at variable pressure and with a constant flow rate.
The invention also relates to the method of rehabilitation of an existing water purification unit of the so-called sand basin type, comprising a basin provided with a bottom floor, an intermediate floor on which the sand bed stands, a raw water inlet, a high trench for the evacuation of cleaning sludges, characterized in that it comprises stages of removal of the sand bed, of destruction of the intermediate floor, of installation on the bottom floor of a series of membrane ultrafiltration units, the membranes being disposed in containers and the operating pressure of these membranes being created by the height of waste water stored in the basin above these membranes.
The invention also an installation for the filtration of water by membranes, comprising a raw water inlet, a connection to a drain, an outlet means for produced water, the membranes being immersed in a filtration volume filled with water to be filtered, whose height of water above the said membranes is adapted to create a differential pressure sufficient to provoke the filtration through these membranes, in which the membranes are of the fibre type with outer skin substantially disposed in a U-shape, whose two open ends are located at the bottom, the potting being carried out at the low point of the said membranes, characterized in that the membranes are disposed in cylindrical containers.